ACUTE CORONARY SYNDROME - POSTERIOR STEMI

Transcription

1 ACUTE CORONARY SYNDROME - POSTERIOR STEMI Black on Gray, oil on canvas, 1969, Mark Rothko, Tate Gallery, London - an unknown adventure into an unknown space.

2 When the Sun is shining on the surface at a very shallow angle, the craters cast long shadows - and the Moon s surface seems very inhospitable, formidably almost. I did not sense any great invitation on the part of the Moon for us to come into its domain. I sensed more a hostile place - a scary place. Michael Collins, Apollo 11. In July of 1969, Michael Collins, it was said, was the loneliest man in history. The Moon is tidally locked to the Earth in its orbit around the Earth. This means that one side permanently faces the Earth - the other is permanently obscured from our vision. After leaving Armstrong and Aldrin on the surface of the Moon, Collins continued in orbit to the fabled dark side of the Moon. Totally alone in the Universe, cut off from all communication with humanity - uncertain if his comrades were alive or dead. Below him he saw a scary place - a vast dark grey vista meeting a horizon of the purest black of the infinite void of space - the darkest vision of some forbidding Rothko place of imagined doom. All of the Apollo astronauts who made the trip to the Moon in the late 1960s and early 1970s were deeply and profoundly affected by their experience. Years later in Ron Howard s masterful documentary, In the Shadow of the Moon, as old men they reminisced their experiences. Collins was often asked how he felt - cut off from humanity on the far side of the Moon - he admitted a certain apprehension - he acknowledged a general anxiety - but surprisingly not loneliness - but rather the shear exhilaration of being a part of mankind s greatest adventure. These men were all made of the right stuff - Collins later explained: I discovered later that I was described as the loneliest man ever in the Universe...or something. Which really is a lot of baloney. I mean I had mission control yakking in my ear half the time. I rather enjoyed it. I certainly was aware of the fact that I was by myself, particularly when I was over on the far side of the Moon. I can remember thinking - God look over there and there s 3 billion people...and two...somewhere down there - and then over here there s one, plus God only knows what; yes I felt that strongly, but I didn t feel it as loneliness, I certainly didn t feel it as fear, I felt it as an awareness, almost a feeling of exaltation, I liked it - it was a good feeling! A very powerfully common leitmotif of the reminiscences of these men was the incredible insight they all say they gained into the vastness of the Universe, and the total insignificance of the Earth we inhabit in relation to it, and from this realization an appreciation of the miracle of an oasis of life that the Earth is and the unbelievable privilege it is to be a sentient part of the Universe. Their sentiments were often expressed in quiet - even tearful - moments of deep reflection - it makes for powerful and compelling viewing. Collins himself of course is one of the very few men to have observed the totality of the Earth as a sphere... How peaceful and calm and quiet and serene it looked. How fragile it appeared; the overriding sensation I got looking at the Earth was - my God - that little thing is so fragile out there!. Many would undergo epiphanies that would change their lives and their views of their place in the Universe forever. David Scott of Apollo 15 rates this heightened awareness of our true place in a vast and incredibly hostile Universe as one of the greatest achievements of the Apollo missions - he found a new deep respect for the Earth that so

3 many of us take for granted, exploit and treat with complete indifference - even a bored contempt. Edgar Mitchell of Apollo 14 related: The joy was on the way home. In my cockpit window every 2 minutes was the Earth, the Moon, the Sun and the whole 360 degree panorama of the heavens - it was a powerful, overwhelming experience. And suddenly I realised, the molecules of my body, and molecules of the spacecraft, and molecules of the bodies of my partners, where prototyped and manufactured in some ancient generation of stars; and that was an overwhelming sense of oneness, of connectiveness. It wasn t them and us - it was that s me - that all of it - and it came accompanied by an...ecstasy...oh my God, wow, yes an insight...an epiphany While some explained their feelings in terms of a deep sense of oneness with the Universe, others experienced feelings of almost religious ecstasy. Eugene Cernan, the last man to walk on the Moon related: I felt that I was literally standing on a plateau somewhere out there in space, a plateau that science and technology had helped me to get to. But now what I was seeing, even more importantly what I was feeling at that moment in time, science and technology had no answers for, because there I was, there you are, the Earth, dynamic, overwhelming - and I felt that world had just too much purpose, too much logic, it was just too beautiful to have happened by accident. There has to be somebody bigger than you and bigger than me, and I mean this in a spiritual sense, not in a religious sense, there has to be a creator of the Universe who stands above the religions that we ourselves create to govern our lives. Jim Lovell, commander of the ill fated Apollo 13 mission, reminisced that from the Moon, you could cover the entire Earth with your thumb, everything you ever knew hidden under a single phalanx, echoing Collins s sense of the extreme vulnerability - the fragility of the Earth - hanging there in infinite space. Alan Bean of Apollo 12 related: I always thought of myself as one of the more fearful astronauts, really - and when I looked out the window - if that window blows out, I m going to die in about a second - there s death right out there, about an inch a way!... Why do people complain about the Earth? - We are living in a Garden of Eden...I never complain about the weather or about too many people...i feel blessed every single day, not a day goes by that I think, this was great, this was wonderful - somebody had to go - and they happened to pick me! Mark Rothko once described his works as an unknown adventure into an unknown space. When our patients present with an ACS, our routine technology does not allow us to see the dark side of the Moon - it s far or posterior aspect remains hidden from our usual view. We must like Michael Collins take a Rothkoesq journey into an unknown space for an unknown adventure - we do this by the modified application of our technology - the posterior chest leads.

4 Introduction ACUTE CORONARY SYNDROME - POSTERIOR STEMI Posterior infarction is one of the most commonly missed MI ECG patterns, as the standard 12 lead ECG does not directly image the posterior walls of the ventricles. Isolated or pure posterior wall infarction is uncommon, (Chan 179) but not rare as was once thought. Infarction of the posterior wall most commonly occurs in association with MI of either the inferior or lateral walls, (Chan 179) Posterior extension of an inferior or lateral infarct is important to recognize as it implies a larger area of myocardial damage, with a consequent increased risk of left ventricular dysfunction and death. Standard 12 lead ECG findings of posterior wall infarction are essentially the mirror images of typical anterior STEMI changes, and diagnosis is confirmed by additional posterior chest leads. Anatomy In left dominant circulations, the left circumflex artery terminates at the crux to become the Left Posterior Descending Artery (LPDA), that supplies the posterior wall of the ventricles. In right dominant circulations, the RCA gives off the Right Posterior Descending Artery (RPDA) that supplies the posterior wall of the ventricles. Pathophysiology Anterior ST segment depression in ACS may be due to: Reciprocal changes to ST segment elevation at the inferior wall. True posterior STEMI or Infero-lateral posterior STEMI Non STEMI/ unstable angina As with STE the ST depression is measured from the J point. The J point is defined as the end of the QRS complex and where the ST segment begins, as shown above.

5 Clinical assessment Clinical assessment is the same as for any patient suspected of having an ACS The clinical presentation of pure posterior infarction is not different from other myocardial infarctions, but the absence of traditional electrocardiographic infarct signs such as ST-segment elevation leads to errors or delay in the diagnosis. Posterior extension of an inferior or lateral infarct is also important to recognize as it implies a larger area of myocardial damage, with a consequent increased risk of left ventricular dysfunction and death. Any patient with an inferior or lateral STEMI, could also have posterior extension and ST segment depression should be carefully looked for. Investigations Blood tests FBE U&Es/ glucose Troponin levels ECG The standard 12 lead ECG does not directly image the posterior walls of the ventricles. Standard 12 lead ECG findings of posterior wall infarction are essentially the mirror images of typical anterior STEMI changes, and include: ST segment depression, V1-3; typically with horizontal morphology. Tall, broad R waves (> 30ms) or R/S ratio > 1 in V1-2, (later development). Upright T waves, V1-2, (later development). Additional evidence of infarction is provided by: Associated ST segment elevation laterally or inferiorly The presence of STE on additional posterior chest leads, i.e. V7-9 as follows : V7: Left posterior axillary line, in the same horizontal plane as V6. V8: Tip of the left scapula, in the same horizontal plane as V6.

6 V9: Left paraspinal region, in the same horizontal plane as V6 Posterior Chest leads V7, V8, V9 Note that ST segment elevation of > 0.5 mm is significant in posterior leads, (as the recording leads are further from the myocardium, compared to the anterior chest leads and so changes are not as prominent) CXR This will not assist in the diagnosis of posterior infarction, but may help rule out differential diagnoses or secondary complications, such as cardiac failure. Echocardiography An echocardiogram is useful in confirming new posterior/ postero-lateral wall motion abnormalities Management Isolated posterior infarction is still an indication for urgent coronary reperfusion. The lack of obvious ST elevation in this condition means however that the diagnosis is often missed, but can be confirmed with additional posterior chest leads Other treatment is given in the ED as appropriate for any other STEMI.

7 Appendix 1 Typical appearance of an infero-lateral STEMI, with associated posterior extension. Posterior leads of the above patient confirm posterior infarction, (extremely well in this case posterior changes are often more subtle than this!), (Life in the Fast Lane Website, ECG Library).

8 References 1. Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby E.O.F. van Gorselen et al. Posterior Myocardial Infarction: the dark side of the moon Case report and review of electrocardiographic diagnosis, Netherlands Heart Journal, Volume 15, Number 1, January 2007, p Dr J. Hayes Dr P. Jordan March 2013